Atomic probe

3D representation of atom probe data of a Cu / Ni-Fe multilayer system. On closer inspection, one can see horizontal grid levels in the Cu (green).

The atom probe is a material science analyzer . It enables the identification of the mass of individual atoms, which are detached from a very sharp point made of an electrically conductive material by means of field evaporation . If it is possible to obtain three-dimensional data of many atoms by using a suitable detector, then they are also called tomographic atom probe ( TAP ) or three-dimensional atom probe (3DAP). Although there are also atomic probes that only allow one-dimensional analysis, what is usually meant is the tomographic atomic probe.

functionality

The TAP can be understood as a further development or addition to the field ion microscope ; the experimental setup is similar. A very sharp tip of an electrically conductive material with a tip radius on the order of 10 to 100 nm is produced electrochemically or by means of a fine ion beam system (FIB). Under ultra-high vacuum conditions and at temperatures at the tip of 20 to 100 K , an electric field with a voltage of 2 to 15 kV is applied, the field strength of which is not sufficient to cause the atoms to detach ( field evaporation ) from the tip. In addition to this base voltage, a very short voltage pulse in the order of magnitude of 10 to 25% of the base voltage is given so that the field strength is briefly sufficient to enable field evaporation of individual atoms. Alternatively, a short laser pulse can be used. The pulses are so short that on average only one atom is detached every 10–100 pulses. If the number of detached atoms is too low or too high, the base voltage is changed in the course of the measurement. The atom, detached as a positively charged ion, is directed to a detector by the electric field. Since the time at which it was detached (the time of the last pulse) is known, the mass of the atom can be determined from the time of flight (as with other time-of-flight mass spectrometers ). The x and y position of the atoms can be determined from the arrival point on the detector. The order in which the atoms arrive is used to determine the z-position. Atoms arriving later were lower within the tip than atoms arriving earlier. In addition to this simple principle, corrections must be made, which are due to the tip geometry (usually assumed to be hemispherical).

The determination of the position in the z-direction is precise enough to be able to differentiate between individual network levels in the reconstructed data (a few hundredths of a nanometer). The resolution in the x and y directions is a few tenths of a nanometer.

A total of several million atoms can be measured, with the latest atom probes, which are particularly characterized by large detector transmission,> 500 million atoms per measurement are possible. This corresponds to a volume of several tens of hundreds of cubic nanometers .

application areas

Some areas of application of the TAP are (not a complete list)

• Multi-layer systems (e.g. TMR , GMR systems)
• Clusters , excretions, especially in the early stages, that cannot be analyzed with transmission electron microscopy
• Regulation
• Stoichiometry of small-scale materials

In principle, only materials with a certain electrical conductivity can be examined using electrical pulses. Less conductive materials and insulators can also be examined using laser pulses.

Web links

• http://www.inspico.de/Theoretical-Background/Atom-Probe-Tomography/
• Article on how the atomic probe works
• Page about the atom probe at ETH Zurich
• Page about the atom probe at the Max Planck Institute for Iron Research in Düsseldorf
• Cameca (producer of atomic probes)

Individual evidence

1. ↑ John W. Valley et al. NATURE GEOSCIENCE | VOL 7 ​​| MARCH 2014 , Hadean age for a post-magma-ocean zircon confirmed by atom-probe tomography.